This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Many membrane proteins are oligomeric and biological activity of peptide antibiotics, such as lipopeptaibols, is usually attributed to their propensity to aggregate, forming different types of channels and pores and by doing so, causing membrane leakage. However, to study aggregation is difficult. Analytical centrifugation is used for proteins, but it is not suited for peptides. Pulse dipolar spectroscopy offers convenient approach to the problem, The intensity of dipolar signal in a group of spins (cluster) is a product function averaged over participating spins, therefore dipolar signal is increasing with the aggregation number. In DEER the dipolar signal referred to zero dipolar evolution time is used to estimate the number of spins in the cluster. The signal is calibrated with the solution of a biradical, serving as a reference aggregation number of two. An aggregate of spin-labeled peptides or protomers can thus be studied with this method. Multiple-quantum coherence spectroscopy offers, in principle more robust method to study aggregation based on separation of coherent orders, but it is yet to be implemented in ESR due to severe technical problems. With the proper amount of care DEER can often provide rather accurate numbers, as was supported in model systems.